ROLLING OF ADVANCED HIGH STRENGTH STEELS FOR AUTOMOTIVE INDUSTRY. Michał DZIEDZIC, Stanisław TURCZYN

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1 ROLLING OF ADVANCED HIGH STRENGTH STEELS FOR AUTOMOTIVE INDUSTRY Michał DZIEDZIC, Stanisław TURCZYN Abstract Akademia Górniczo-Hutnicza, al. Mickiewicza 30, Kraków, Poland The group of steels meeting very high automotive requirements includes the newer type called Advanced High Strength Steels (AHSS), which usually comprises Dual Phase (DP), Transformation Induced Plasticity (TRIP), Complex Phase (CP) and Martensitic Steels (MS). The paper focuses on hot rolled and fast cooled AHSS steel strips for automotive application. The analysis carried out in this work includes many tests and experiments such as: dilatometric and plastometric tests, Gleeble simulations, FEM calculation of rolling and hot rolling of steel samples in laboratory conditions. As the example, for the detail analysis the DP steel has been chosen. The major part of the paper contains the results of experimental rolling of DP steel samples in laboratory conditions. Especially, the influence of cooling rate on phase volume fraction and some mechanical properties of DP strips has been analysed. The realized hot rolling with controlled cooling of DP samples in laboratory conditions allowed for obtaining diversified steel microstructure and thus wide range of mechanical properties. The results of the analysis carried out in this work provide the useful data for the designing of thermo-mechanical rolling of DP steel strips or adjusting existing processes to meet very high requirements demanded by the automotive industry. Keywords: AHS steels, physical simulation, thermo-mechanical rolling, mechanical properties, DP steels 1. ADVANCED HIGH STRENGTH STEELS Automotive industry requires steel producers to continuously accommodate the production to the consumer s demands. Practically, it brings to a compromise between high cold-workability of steel sheet and rigidity of a car body draw-piece. In another words, these steels are designed to reduce density, increase strength and improve elongation. The group of steels meeting very high automotive requirements include the newer type called Advanced High Strength Steels (AHSS), which usually comprises Dual Phase (DP), Transformation Induced Plasticity (TRIP), Complex Phase (CP) and Martensitic Steels (MS). Forecast contribution of these steels to vehicle manufacture of reduced mass is shown in Fig. 1. The metallurgy and processing of advanced high-strength steels is generally well known, but is somewhat different compared to conventional steels. All these steels are produced by controlling the cooling rate after hot rolling (on runout table) or cold rolling (in annealing furnace). Generally rolling of AHS steels do not brings new problems, but special attention can be made on higher loads during deformation, grater energy requirements, springback compensation, etc. In this paper DP steel has been chosen to represent AHSS group in the analysis and experimental testing, which results are presenting in following chapters. 2. METALLURGY AND PROCESSING OF DUAL PHASE STEELS DP steels are a group of low-carbon micro-alloyed steels, whose microstructure consists of soft ferritic matrix, in which % of martensite is distributed, Fig. 2. Depending on the process route and steel composition, hot rolled strips can have a microstructure containing some quantities of bainite. DP steels show very

2 high strength as well as ductility, high level of homogeneous strain, good formability, weldability and excellent absorption of mechanical energy during high-speed deformation [1, 8]. Depending on martensite volume fraction, the tensile strength (R m ) ranges from 500 to about 1000 MPa, and total elongation amounts to %. The ferritic-martensitic microstructure of these steels results in adequately low yield stress (R e ) and high ultimate tensile strength (R m ), allowing for obtaining sufficiently low R e /R m ratio. Soft ferrite facilitates the beginning of plastic deformation, while hard martensite increases the strength of steel [2, 3]. DP steels show high strain-hardening coefficient n, which determines the maximum allowable stretch of sheets. A higher n- value compared to a lower one means a deeper part can be stretched. Microstructural internal stresses occurring during martensite formation increase the density of mobile dislocations that facilitate the beginning of plastic deformation and prevent from the occurrence of discontinuities at the physical yield point. DP steels also show continuous workability with no need to perform temper rolling, the occurrence of BH effect after preliminary deformation as well as low value of the planar anisotropy coefficient of sheets. In spite of excellent properties of DP steels, the automotive industry takes advantage of them only to a small degree [5, 6]. BH - 10% 10% 4% 4% 3% 1% 3% 1% HSLA - 1% DP - 74% IF - 4% 74% TMS - 4% TRIP - 3% CP - 1% other - 3%? ferrite? martensite Fig. 1. Forecast contribution of steel grades to the production of a modern car [8] Fig. 2. Islands of martensite (black) in a matrix of ferrite in DP steel The prevailing technology of DP steel strip production consists of the following processes: hot rolling as the thermo-mechanical process, cold rolling (after hot rolling) with recrystallization annealing in the two-phase region ( + ) and controlled, fast cooling from this region to the temperature of martensite transformation, which aims at avoiding pearlite reaction. However, the technology that seems to be prevailing in the future is hot strip rolling realized in modern rolling mills integrated with continuous casting of thin ingots (slabs). 3. NUMERICAL AND PHYSICAL SIMULATION OF HOT ROLLING 3.1 Plastometric and Gleeble tests The samples for all tests were prepared from special cast of DP steel (cast A1), which was made in laboratory condition. Chemical composition of DP steel was designed according to the standard EN 10336, Tab. 1. In order to improve hardenability, it contains increased manganese and silicon content. As the elements facilitating the obtaining of ferritic-martensitic microstructure after recrystallization annealing, additions of chromium and molybdenum was also applied. Tab. 1. Chemical composition of the investigated DP steel (cast A1) C Mn Si Cr Ni Mo Al The cooling curves and the start/ finish temperatures of phase transformations (critical temperatures) were obtained using physical simulation of a cooling process in which dilatometer (DT1000 type) was utilized [4, 7]. In order to construct CCT diagram the specimens from DP steel were austenitized in the temperature above Ac 3, and subsequently cooled at different cooling rates, while recording changes in specimen s length

3 Flow stress, MPa , Brno, Czech Republic, EU as a function of temperature. The precise CCT curves [4, 7] were the basis to the design of technology allowing for obtaining the most favourable material properties, considering further processing. Prior to computer calculations a series of tests were performed using torsional plastometer. The obtained results in a form of flow stress variations as a function of temperature, strain and strain rate. They were loaded into computer program as the material database. The example flow stresses for DP steel (cast A1) are presented in Fig Steel grade - DP type ,00 0,44 0,87 1,31 1,74 2,18 True strain Fig. 3. Flow stresses for DP steel obtained from torsion tests for various temperatures (in C) and strain rate equal 2 s -1 Physical modelling of the process of hot rolling in six final passes, as in industrial process, was realized with application of Gleeble 3800 simulator. The test parameters, i.e. temperature of rolled strip, strain, strain rate and idle time between passes were selected to represent the deformation conditions occurring in real process as precisely as possible. Example results, obtained for rolling simulation, are shown in Fig. 4. The Gleeble simulations also confirmed results obtained from dilatometric tests. Fig. 4. Simulation of strip rolling in six final passes using Gleeble 3800 (final temperature 890 C and final strain rate of about 100 s -1 ) The performed plastometric tests, including Gleeble simulations, allowed for determining the changes of flow stress as a function of temperature, strain, and strain rate. Prior to further calculations and tests, these relationships were stored in the material database of finite element method (FEM) program.

4 Volume fraction, % Re, Rm, MPa, HB , Brno, Czech Republic, EU 3.2 Simulation of strip cooling Numerical methods were used to help a proper phase engineering during thermo-mechanical rolling. In order to obtain evaluation of the roll-end temperatures of the strip, the simulation of hot strip rolling with application of computer program (FormFEM/ROLL) has been applied [4]. The results of calculations contribute also to better understanding of flow pattern of a strip in the roll stands, as well as the distributions of temperature, stresses and strains in a strip being deformed. For simulation of strip cooling after hot rolling commercial software (TTSteel) was used. Cooling rates ranging from 500 C/s to 1 C/s were selected and simulation was performed for cooling from the temperature of 890 C. The chemical compositions of steel as well as cooling temperature-time relationships determined by dilatometer were stored in the program database. The effect of chemical composition on the critical temperatures and time of phase transformations was taken into account basing on the following equations: T ( i) A0 i) (1) A( i) c( S ( i) exp{ B0 i } (2) where: B( i) c( ) T critical temperature of the i-th transformation, S time of the i-th transformation, A 0, B 0 regression constants, A(i), B(i) regression coefficients, c(i) content of alloying element. Regression constants and coefficients in Eqs. (1) and (2) were computed by the inner code of the program on the basis of known curves of phase transformations. The effect of cooling rate on phase composition and forecast mechanical properties of dual phase steel is presented in Fig. 5 and Fig ferrite pearlite bainite martensite Re Rm HB Cooling rate, C/s Cooling rate, C/s Fig. 5. Influence of cooling rate on phase volume fractions (according to TTSteel results) Fig. 6. Influence of cooling rate on mechanical properties (according to TTSteel results) The results of calculation confirm that the cooling rate higher than 20 C/s makes it possible to obtain the ferritic-bainitic-martensitic microstructure, while at the cooling rate lower than 10 C/s only the ferritic-pearlitic structure develops. The software allows for determining the critical temperatures and CCT curves for theoretical cooling with different cooling rates. It also allows for prediction phase composition and forecast mechanical properties of investigated steels [3, 4, 7]. All these investigations were the basis to the planning of further research e.g. rolling parameters of DP samples in laboratory condition.

5 4. EXPERIMENTAL ROLLING OF DP STEEL The samples for experimental rolling were prepared from special cast of DP steel (cast A1). The round ingot of diameter 210/190 mm and length 400 mm was cast in laboratory condition. After head and tail cropping it was preliminary hot deformed to the flat specimens of dimensions 12.4 x 26.8 x 120 mm. These specimens were used in experimental hot rolling and cooling, which schedule is presented in Tab. 2. Tab. 2. Schedule of experimental rolling and controlled cooling of dual phase steel samples Series Heating temp., C HOT ROLLING Roll end Number temperature of passes Reductions in the passes, % COOLING Cooling way 2P - A 1250 above Ar x 60 water 2P - B 1250 above Ar x 60 air 3P - C 1250 below Ar x x 35 water 3P - D 1250 below Ar x x 35 water sprinkling 3P - E 1250 below Ar x x 35 air 3P - F 1250 below Ar x x 35 water + holding in ferrite region 5. DISCUSSION OF RESULTS The complexity of processes taking place in hot rolling conditions creates a wide range of possibilities of controlling the microstructure and mechanical properties of thermo-mechanically treated strips. However, the principal deciding factor is the ensuring of controlled cooling rate, from finish-rolling temperature in austenite range to coiling temperature. The realized rolling of DP samples in laboratory conditions together with controlled cooling allowed for obtaining diversified steel microstructures, depending on the roll-end temperatures and the cooling rates, Tab. 3. Particularly, in case of analysed DP steel it was found that cooling at the rate not greater than 100 C/s leads to the ferritic-bainitic microstructure with hardness of about 190 HV, while when applying the cooling rate of 10 C/s the ferritic-pearlitic structure develops. For example, Fig. 7 shows the ferritic-bainitic microstructure of specimen which was cooled at the rate of 50 C/s (hardness of about 180 HV). Fig. 7. Microstructure of DP steel obtained from Gleeble simulations after cooling with rate 50 C/s Some mechanical properties of strip samples obtained after experimental rolling in three passes and controlled cooling with different rate are presented in Fig. 7 and Fig. 8.

6 Tab. 3. Average content of phase fraction in DP steel after thermo-mechanical rolling Series Roll end temperature, C Phase content *), % 2P A 909 F-30.4; M P B 880 F-66.0; P P C 787 F-34.4; M P D 747 F-59.3; B P E 725 F-79.0; P P F 768 F-69.4; M-19.1; B-11.5 *) F - ferrite, M martensite, B bainite Fig. 8. Comparison of tensile strength and yield Fig. 9. Comparison of elongations (A 50 ) strength for strip samples rolled in 3 passes for strip samples rolled in 3 passes The increase of cooling rate after hot rolling above the critical cooling rate results in increased martensite volume fraction in steel, and thus higher strength and lower formability of investigated strip samples. The cooling rates for different coolants (water, water sprinkling, air) were computed after some tests where thermovision camera (ThermaCAM S60) was applied for recording temperature decrease of the samples. Martensitic phase prevails (from 66 to 70 %) when using water cooling (rate somewhat above 100 ºC/s) and thus very high strength (R e, R m, and HV) and low formability (A 50 ) of steel strips were obtained. In case of water sprinkling cooling (rate about 15 ºC/s), apart from ferritic the bainitic phase (41 % for 3P-D samples) was formed. Thus, lower but enough high strength and better formability of strip samples were obtained. When air was used as a coolant (rate about 4 ºC/s), only ferritic-pearlitic microstructures were observed. However, the best results have been obtained when water cooling with holding inside ferrite region (about 7 s) was applied (3P F samples). In this case hot rolled strips had microstructure containing much lower of martensite (19.1 %) and some quantities of bainite (11.5 %). The obtained microstructure results in adequately low yield stress (R e = 479 MPa) and high ultimate strength (R m = 786 MPa), allowing for obtaining sufficiently good R e /R m ratio (equal 0.61) and acceptable level of cold formability (A 50 = 15 %). By combining a number of different microstructures a wide range of mechanical properties of DP steel strips were possible for obtaining. This shows that rolling mills can adjust processing of DP strips to meet the properties requirements demanded by the automotive industry. 6. CONCLUSIONS The complexity of industrial hot rolling process usually requires more detailed investigations than were made in this work. However, the results of the analysis provide useful data for the designing of thermo-mechanical

7 rolling of DP steel strips or adjusting existing processes to meet very high requirements demanded by the automotive industry. For example, they can be also suitable for operation of the new hot strip mill L=2250 mm in Cracow. The obtained results also allow for formulating the more general conclusions: 1. The analysis of strip cooling after hot rolling with application of dilatometric tests and TTSteel program allows evaluating important process parameters, used in further research. The obtained results show that the cooling rate higher than 20 C/s makes it possible to obtain the ferritic-martensitic (or bainitic) microstructure, while at the cooling rate lower than 10 C/s only the ferritic-pearlitic microstructure develops. 2. The most satisfactory results of experimental rolling were obtained when water-cooling with holding inside ferrite region was applied (3P F samples). In this case hot rolled strips had microstructure containing much lower of martensite (19.1 %) and some quantities of bainite (11.5 %). The obtained microstructure results in adequately low yield stress (R e = 479 MPa) and high ultimate strength (R m = 786 MPa), allowing for obtaining very good R e /R m ratio (equal 0.61) and acceptable level of cold formability (A 50 = 15 %). 3. By combining a number of different microstructures a wide range of mechanical properties of DP steel strips are possible for obtaining. This allows rolling mills to adjust process parameters to meet market requirements, especially demanded by the automotive industry. ACKNOWLEDGEMENT Financial assistance of Polish Ministry of Science and Higher Education (AGH Project No ) is acknowledged. LITERATURE [1] BLECK W., PAPAEFTHYMIOU S., FREHN A.: Microstructure and tensile properties in DP and TRIP steels. Proc. of Conf. for Mechatronics, Patras 2004, pp [2] BODIN A., FLEMMING J., JANSEN E.F.M.: Development of as-hot-rolled low-silicon and micro alloyed dualphase steels. Proc. of Conf. 42 nd MWSP, ISS, vol. 38, Ontario 2000, pp [3] DZIEDZIC M., TURCZYN S.: Mechanical properties of dual phase steel strips obtained in thermomechanical rolling process. Hutnik Wiadomości Hutnicze, vol. 73, no. 12, 2006, pp [4] DZIEDZIC M., TURCZYN S.: Thermomechanical rolling of dual phase steel strips. Proc. of 5 th Int. Conf. on Industrial Tools (ICIT), Velenje, Celje 2005, pp [5] REICHERT B., FREIER K.: Stahlwerkstoffe für den modernen Leichtbau. Proc. of Conf. Meform 2003, Freiberg, pp [6] TURCZYN S., DZIEDZIC M.: Rolling of car body sheets made from new generation steel. Hutnik Wiadomości Hutnicze, vol. 69, no. 4, 2002, pp [7] TURCZYN S.: Effect of rolling and cooling of Dual Phase steel on structure and mechanical properties of strips. Hutnik Wiadomo ci Hutnicze, vol. 76 (2009), no. 4, pp [8] Ultra Light Steel Auto Body Advanced Vehicle Technology (ULSAB-AVC) Programme. Date of access: March 2009.